Method and system for electric discharge machining insulating material or high resistance material

ABSTRACT

An electric discharge machine for machining a workpiece ( 2   a ) made of insulating material or other materials comprises: a first comparison member ( 17 ) for comparing voltage (V) between the electrodes of the electrode ( 1 ) and the workpiece ( 2   a ) with the first reference voltage (V 1 ) which is set at a value close to electric power source voltage (V 0 ) not higher than electric power source voltage (V 0 ); a second comparison member ( 18 ) for comparing voltage (V) between the electrodes with the second reference voltage (V 2 ) which is set at a value lower than the first reference voltage (V 1 ); and a control member ( 19 ) for setting a short electric discharge pulse width (T P1 ) in the case where voltage (V) between the electrodes is lower than the second reference voltage (V 2 ) according to the result of comparison conducted by the second comparison member ( 18 ) at a point of time when a predetermined period of time (T 0 ) has passed from the time when voltage (V) between the electrodes becomes lower than the first reference voltage (V 1 ) according to the result of comparison conducted by the first comparison member ( 17 ), and for setting a long electric discharge pulse width (T P2 ) in the case where the voltage between the electrodes is higher than the second reference voltage (V 2 ). This electric discharge machine is capable of stabilizing the machining of the workpiece (2 a ) made of insulating material or other materials and also capable of enhancing quality of a machined face.

TECHNICAL FIELD

The present invention relates to improvements in an electric dischargemachining method and electric discharge machine for machining aworkpiece when machining electric power is supplied between theworkpiece, which is made of insulating material or high resistancematerial, and an electrode so that pulse-like electric discharge isgenerated between the electrodes.

BACKGROUND ART

FIGS. 8A to 8E are schematic illustrations showing a mechanism ofelectric discharge machining. In the drawing, reference numeral 1 is anelectrode, reference numeral 2 is a workpiece, reference numeral 3 is anarc column, reference numeral 4 is a working solution, and referencenumeral 5 represents chips produced in the process of electric dischargemachining. The workpiece 2 is being machined by electric dischargemachining through the cycle from the following items (a) to (e) whichcorrespond to FIG. 8A to FIG. 8E. Each item is described as follows.Item (a) is a process in which the arc column 3 is formed by thegeneration of electric discharge, item (b) is a process in which theworkpiece 2 is locally melted and the working solution 4 is vaporized bythermal energy generated by electric discharge, item (c) is a process inwhich the working solution 4 is vaporized and an explosive force isgenerated by the vaporization of the working solution 4, item (d) is aprocess in which a melted portion (chips 5 generated in the process ofmachining) of the workpiece 2 is scattered, and item (e) is a process inwhich the workpiece is cooled by the working solution 4 and solidifiedso that insulation between the electrodes are recovered. When this cycleis repeated highly frequently, the workpiece 2 can be machined.

The above electric discharge machining acquires a firm position as atechnique of machining metallic dies and is put into practical use inthe fields of automobile industry, electric appliance industry,semiconductor industry and so forth. However, since electric dischargemachining is a method of machining in which the phenomenon of electricdischarge is utilized as shown by the cycle illustrated in FIGS. 8A to8E, it is common that electric discharge machining is applied when theworkpiece 2 made of conductive material such as material of iron ismachined.

Concerning the method of conducting electric discharge machining on theworkpiece 2 made of insulating material, there is provided a methoddisclosed in the official gazette of the Japanese Unexamined PatentPublication No. Sho63-150109. According to the method, a conductive filmis coated onto a surface of insulating material by means of flamecoating or vapor deposition, and then electric discharge is conducted oninsulating material in a working solution containing carbon. For theobject of abolishing a specific processing device for applying the aboveconductive film onto the surface of insulating material, the JapaneseUnexamined Patent Publication Nos. Hei7-136849 and Hei9-253935 disclosea technique in which a workpiece made of insulating material or highresistance material is made to directly come into contact withconductive material and subjected to electric discharge machining in aworking solution containing carbon.

FIG. 9 is a schematic illustration showing a composition of a wireelectric discharge machine disclosed in the Japanese Unexamined PatentPublication No. Hei9-253935. In the drawing, reference numeral 1 a is awire electrode, reference numeral 2 a is a workpiece made of insulatingmaterial or high resistance material, reference numeral 4 a is a workingsolution containing carbon, reference numeral 6 is a table on which theworkpiece 2 a is put, reference numeral 7 is a fixing jig, referencenumerals 8 a, 8 b are working solution nozzles, reference numerals 9 a,9 b are wire guides, reference numeral 10 is a feeder piece, referencenumeral 11 is a working solution supplying pump which is a member forsupplying a working solution, reference numeral 12 is conductivematerial, and reference numeral 13 is a machining electric powersupplying member. The workpiece 2 a and conductive material 12 are fixedbeing joined to each other by the fixing jig 7. The wire electrode 1 ais positioned at a joining portion of the workpiece 2 a and theconductive material 12 by a positioning member not shown in the drawing.When machining electric power is supplied between the wire electrode 1 aand the workpiece 2 a and also between the wire electrode 1 a andconductive material 12 by the machining electric power supply member 13,first, electric discharge is generated only in a portion of theconductive material 12, and then machining is conducted by thermalenergy and a component of the electrode is transferred to the workpiecein a portion of the workpiece 2 a closest to the conductive material 12.After that, electric discharge is also generated in the transferredportion of the workpiece 2 a. In this way, machining of the workpiece 2a proceeds by a shock of electric discharge and thermal energy generatedby electric discharge. According to the progress of electric dischargemachining, the component of the electrode is transferred onto thesurface of the workpiece 2 a to be machined, and the working solution 4a is thermally decomposed. Therefore, carbon contained in the workingsolution 4 a attaches to the workpiece 2 a in the form of crystallinecarbon, the electric resistance of which is relatively low, so that aconductive film can be formed. Electric discharge is generated on theconductive film formed in this way, and the workpiece 2 a is machined.

As described above, the principle of the conventional electric dischargemachining technique in which a workpiece made of insulating material orhigh resistance material is machined by electric discharge is that theworkpiece is machined through a conductive film formed on the workpiece.

However, when the above conventional electric discharge machiningtechnique is adopted, machining can not be stably conducted and furtherquality of a machined surface is not high. For the above reasons, theabove conventional electric discharge technique has not been put intopractical use on a full scale.

DISCLOSURE OF INVENTION

The present invention has been accomplished to solve the above problems.It is an object of the present invention to provide an electricdischarge machining method and machine capable of stably machining aworkpiece made of insulating material or high resistance material andenhancing quality of a machined surface of the workpiece.

The present invention provides an electric discharge machining methodfor machining a workpiece when machining electric power is suppliedbetween the workpiece, which is made of insulating material or highresistance material coated with a conductive film, and an electrode, ormachining electric power is supplied between the workpiece, which ismade of insulating material or high resistance material coming intocontact with conductive material, and an electrode so that pulse-likeelectric discharge is generated between the electrodes in a workingsolution containing carbon, the electric discharge machining methodincluding the steps of a voltage between the electrodes at a point oftime when a predetermined period of time has passed from the start ofelectric discharge is measured, and a relatively short electricdischarge pulse width suitable for machining the workpiece according tothe measured value is set and a relatively long electric discharge pulsewidth suitable for forming a conductive film on a surface of theworkpiece to be machined is set.

The present invention also provides an electric discharge machiningmethod for machining a workpiece when machining electric power issupplied between the workpiece, which is made of insulating material orhigh resistance material coated with a conductive film, and anelectrode, or machining electric power is supplied between theworkpiece, which is made of insulating material or high resistancematerial coming into contact with conductive material, and electrode sothat pulse-like electric discharge is generated between the electrodesin a working solution containing carbon, the electric dischargemachining method including the steps of a voltage between the electrodesat a point of time when a predetermined period of time has passed fromthe start of electric discharge is measured, and a relatively shortelectric discharge pulse width suitable for machining the workpiece inthe case where the measured value is lower than a predeterminedreference voltage is set, and a relatively long electric discharge pulsewidth suitable for forming a conductive film on a face of the workpieceto be machined in the case where the measured value is higher than apredetermined reference voltage is set.

The present invention also provides an electric discharge machiningmethod for machining a workpiece when machining electric power issupplied between the workpiece, which is made of insulating material orhigh resistance material coated with a conductive film, and anelectrode, or machining electric power is supplied between theworkpiece, which is made of insulating material or high resistancematerial coming into contact with conductive material, and electrode sothat pulse-like electric discharge is generated between the electrodesin a working solution containing carbon, the electric dischargemachining method including the steps of at least one reference voltage,which is lower than an electric power source voltage, with a voltagebetween the electrodes is compared, and a relatively short electricdischarge pulse width suitable for machining the workpiece according tothe result of the comparison is set, and a relatively long electricdischarge pulse width suitable for forming a conductive film on asurface of the workpiece to be machined according to the result of thecomparison is set.

The present invention also provides an electric discharge machiningmethod for machining a workpiece when machining electric power issupplied between the workpiece, which is made of insulating material orhigh resistance material coated with a conductive film, and anelectrode, or machining electric power is supplied between theworkpiece, which is made of insulating material or high resistancematerial coming into contact with conductive material, and electrode sothat pulse-like electric discharge is generated between the electrodesin a working solution containing carbon, the electric dischargemachining method including the steps of a voltage between the electrodesat a point of time when a predetermined period of time has passed fromthe time when the voltage between the electrodes becomes lower than afirst reference voltage which is set at a value close to the electricpower source voltage lower than the electric power source voltage ismeasured, and a relatively short electric discharge pulse width suitablefor machining the workpiece in the case where the measured value islower than a predetermined second reference voltage is set, and arelatively long electric discharge pulse width suitable for forming aconductive film on a face of the workpiece to be machined in the casewhere the measured value is higher than a predetermined second referencevoltage is set.

The present invention also provides an electric discharge machine formachining a workpiece by generating pulse-like electric dischargebetween an electrodes including a machining electric power supply memberfor supplying machining electric power between a workpiece made ofinsulating material or high resistance material which are coated with aconductive film, and an electrode, or a machining electric power supplymember for supplying machining electric power between the workpiece madeof insulating material or high resistance material which are coming intocontact with conductive material, and an electrode, a machining liquidsupply member for supplying a machining liquid containing carbon betweenthe electrodes, a positioning member for relatively positioning theworkpiece and the electrode, a voltage measuring member for measuringvoltage between the electrodes at a point of time when a predeterminedperiod of time has passed from the start of electric discharge, and acontrol member for setting a relatively short electric discharge pulsewidth suitable for machining the workpiece according to the measuredvalue of voltage between the electrodes measured by the voltagemeasuring member for measuring voltage between the electrodes and forsetting a relatively long electric discharge pulse-width suitable forforming a conductive film on a surface of the workpiece to be machined.

The present invention also provides an electric discharge machine formachining a workpiece by generating pulse-like electric dischargebetween an electrodes including a machining electric power supply memberfor supplying machining electric power between a workpiece made ofinsulating material or high resistance material which are coated with aconductive film, and an electrode, or a machining electric power supplymember for supplying machining electric power between the workpiece madeof insulating material or high resistance material which are coming intocontact with conductive material, and an electrode, a machining liquidsupply member for supplying a machining liquid containing carbon betweenthe electrodes, a positioning member for relatively positioning theworkpiece and the electrode, a voltage measuring member for measuringvoltage between the electrodes at a point of time when a predeterminedperiod of time has passed from the start of electric discharge, and acontrol member for setting a relatively short electric discharge pulsewidth suitable for machining the workpiece in the case where themeasured value is lower than a predetermined reference voltage accordingto the measured value of voltage between the electrodes measured by thevoltage measuring member for measuring voltage between the electrodes,and for setting a relatively long electric discharge pulse widthsuitable for forming a conductive film on a face of the workpiece to bemachined in the case where the measured value is higher than apredetermined reference voltage according to the measured value ofvoltage between the electrodes measured by the voltage measuring memberfor measuring voltage between the electrodes.

The present invention also provides an electric discharge machine formachining a workpiece by generating pulse-like electric dischargebetween an electrodes including a machining electric power supply memberfor supplying machining electric power between a workpiece made ofinsulating material or high resistance material which are coated with aconductive film, and an electrode, or a machining electric power supplymember for supplying machining electric power between the workpiece madeof insulating material or high resistance material which are coming intocontact with conductive material, and an electrode, a machining liquidsupply member for supplying a machining liquid containing carbon betweenthe electrodes, a positioning member for relatively positioning theworkpiece and the electrode, a voltage measuring member for measuringvoltage between the electrodes, and a control member for setting arelatively short electric discharge pulse width suitable for machiningthe workpiece in the case where a measured value (second measuredvalue), which is measured by the voltage measuring member for measuringvoltage between the electrodes at a point of time when a predeterminedperiod of time has passed from the time when a measured value (firstmeasured value) measured by the voltage measuring member for measuringvoltage between the electrodes becomes a value lower than a firstreference voltage which is set close to electric power source voltagenot higher than the electric power source voltage, is lower than apredetermined second reference voltage, and for setting a relativelylong electric discharge pulse width suitable for forming a conductivefilm on a face of the workpiece in the case where the second measuredvalue is higher than the predetermined reference voltage.

The present invention also provides an electric discharge machine formachining a workpiece by generating pulse-like electric dischargebetween an electrodes including a machining electric power supply memberfor supplying machining electric power between a workpiece made ofinsulating material or high resistance material which are coated with aconductive film, and an electrode, or a machining electric power supplymember for supplying machining electric power between the workpiece madeof insulating material or high resistance material which are coming intocontact with conductive material, and an electrode, a machining liquidsupply member for supplying a machining liquid containing carbon betweenthe electrodes, a positioning member for relatively positioning theworkpiece and the electrode, at least one comparison member forcomparing a reference voltage, which is lower than the electric powersource voltage, with the voltage between the electrodes, and a controlmember for setting a relatively short electric discharge pulse widthsuitable for machining the workpiece according to the result of thecomparison, and for setting a relatively long electric discharge pulsewidth suitable for forming a conductive film on a face of the workpieceto be machined.

The present invention also provides an electric discharge machine formachining a workpiece by generating pulse-like electric dischargebetween an electrodes including a machining electric power supply memberfor supplying machining electric power between a workpiece made ofinsulating material or high resistance material which are coated with aconductive film, and an electrode, or a machining electric power supplymember for supplying machining electric power between the workpiece madeof insulating material or high resistance material which are coming intocontact with conductive material, and an electrode, a machining liquidsupply member for supplying a machining liquid containing carbon betweenthe electrodes, a positioning member for relatively positioning theworkpiece and the electrode, a comparison member for comparing voltagebetween the electrodes at a point of time when a predetermined period oftime has passed from the start of electric discharge with predeterminedreference voltage, and a control member for setting a relatively shortelectric discharge pulse width suitable for machining the workpiece inthe case where the voltage between the electrode is lower than thereference voltage according to the result of the comparison conducted bythe comparison member and a control member for setting a relatively longelectric discharge pulse width suitable for forming a conductive film ona face of the workpiece in the case where the voltage between theelectrode is higher than the reference voltage.

The present invention also provides an electric discharge machine formachining a workpiece by generating pulse-like electric dischargebetween an electrodes including a machining electric power supply memberfor supplying machining electric power between a workpiece made ofinsulating material or high resistance material which are coated with aconductive film, and an electrode, or a machining electric power supplymember for supplying machining electric power between the workpiece madeof insulating material or high resistance material which are coming intocontact with conductive material, and an electrode, a machining liquidsupply member for supplying a machining liquid containing carbon betweenthe electrodes, a positioning member for relatively positioning theworkpiece and the electrode, a first comparison member for comparingvoltage between the electrodes with a first reference voltage which isset at a value close to the electric power source voltage not higherthan the electric power source voltage, a second comparison member forcomparing voltage between the electrodes with a second reference voltagewhich is set at a value lower than the first reference voltage, and acontrol member for setting a relatively short electric discharge pulsewidth suitable for machining the workpiece in the case where the voltagebetween the electrodes is lower than the second reference voltageaccording to the result of the comparison conducted by the secondcomparison member at a point of time when a predetermined period of timehas passed from the time at which the voltage between the electrodesbecomes lower than the first reference voltage according to the resultof the comparison conducted by the first comparison member, and forsetting a relatively long electric discharge pulse width suitable forforming a conductive film on a machined face of the workpiece in thecase where the voltage between the electrodes is higher than the secondreference voltage.

The electric discharge machining method and electric discharge machineof the present invention is composed as described above. Therefore, itis possible to realize stabilization of machining a workpiece made ofinsulating material or high resistance material and enhancement ofquality of a machine face.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration showing an example of a wave-form ofvoltage impressed between electrodes of a conventional electricdischarge machine by which a workpiece made of insulating material orhigh resistance material is machined.

FIG. 2 is a schematic illustration showing a composition of an electricdischarge machine of Embodiment 1 of the present invention.

FIG. 3 is a schematic illustration showing an example of a wave-form ofvoltage impressed between electrodes of an electric discharge machine ofEmbodiment 1 of the present invention.

FIGS. 4A and 4B are schematic illustrations showing a phenomenon ofelectric discharge generated in the process of machining a workpiecemade of insulating material or high resistance material.

FIGS. 5A and 5B are schematic illustrations showing a wave-form ofvoltage impressed between electrodes corresponding to FIGS. 4A and 4B.

FIG. 6 is a schematic illustration showing an example of a change in awave-form of voltage impressed between electrodes, wherein the change inthe wave-form of voltage is caused by a state of a conductive film.

FIG. 7 is a schematic illustration showing a composition of an electricdischarge machine of Embodiment 2 of the present invention.

FIGS. 8A to 8E are schematic illustrations showing a mechanism ofelectric discharge machining.

FIG. 9 is a schematic illustration showing a composition of aconventional wire electric discharge machine by which a workpiece madeof insulating material or high resistance material is machined.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

FIG. 1 is a schematic illustration showing an example of a wave-form ofvoltage impressed between electrodes of a conventional electricdischarge machine by which a workpiece made of insulating material orhigh resistance material is machined. In the drawing, reference mark tis time, reference mark V is voltage impressed between electrodes,reference mark V₀ is electric power source voltage, reference mark Vg isarc voltage, reference mark Vs is electric discharge detection voltage,reference mark T_(P) is a predetermined electric discharge pulse width,reference mark T_(PL) is an electric discharge pulse width longer thanelectric discharge pulse width T_(P) which appears at a certainfrequency, and reference mark Tr is recess time.

In the case where electric discharge machining is conducted by aconventional electric discharge machine, by which a workpiece made ofinsulating material or high resistance material is machined, while aconstant electric discharge pulse width T_(P) is set so as to obtain amachined face of uniform surface roughness by keeping electric dischargeenergy at a constant value, when a state of electric discharge machiningis observed, it is possible to observe electric discharge pulse widthT_(PL) which is longer than electric discharge pulse width T_(P) asshown in FIG. 1. The above wave-form of voltage impressed betweenelectrodes will be studied as follows.

A commonly used electric discharge machine is planned as follows. Inorder to make an electric discharge pulse width to be a predeterminedelectric discharge pulse width T_(P), a predetermined voltage not higherthan electric power source voltage V₀ is used as electric dischargedetection voltage V_(S), and at a point of time when voltage becomeslower than electric discharge detection voltage V_(S), it is recognizedthat electric discharge has started, and the measurement of electricdischarge pulse width T_(P) is started. However, in the case of electricdischarge machining of a workpiece made of insulating material or highresistance material, since electric resistance on a surface of theworkpiece is high, voltage between electrodes impressed after the startof electric discharge becomes high. In this case, the voltage betweenelectrodes includes a drop in voltage caused by electric resistance ofthe workpiece. Therefore, as shown by A in FIG. 1, voltage V betweenelectrodes is not lower than electric discharge detection voltage V_(S)although electric discharge is started. Therefore, the electricdischarge machine does not recognize that electric discharge hasstarted. Accordingly, the electric discharge machine keeps on impressingvoltage. In this case, voltage between electrodes is gradually lowered.After a period of time corresponding to a predetermined electricdischarge pulse width T_(P) (for example, several 10 μs) has passed froma point of time when voltage V between electrodes gradually decreasesand becomes lower than electric discharge detection voltage V_(S), theimpression of voltage is stopped and the process comes to rest forrecess time Tr. As described above, a phenomenon is caused in which anactual pulse width becomes T_(PL) that is several times or several tenstimes as long as the above predetermined electric discharge pulse widthT_(P).

The phenomenon itself can be said to be a phenomenon of malfunctioncaused in the case where a workpiece made of insulating material or highresistance material is machined by an electric discharge machine whichis manufactured in order to machine a workpiece made of conductivematerial such as steel. However, the above phenomenon fulfills animportant function when a workpiece made of insulating material or highresistance material is machined by electric discharge machining. Thatis, when an electric discharge pulse, the pulse width of which is muchlonger than the pulse width of the setting, appears according to theabove phenomenon, it becomes possible to machine insulating material orhigh resistance material. As shown in the description of the backgroundart, when a workpiece made of insulating material or high resistancematerial is machined by electric discharge machining, it is necessary toconduct machining while a conductive film is being formed on a surfaceof the workpiece. According to the investigation made by the presentinventors, the following facts have been made clear. An electricdischarge pulse, the pulse width of which is short and the same as thesetting value (This pulse is referred to as “a short electric dischargepulse” hereinafter.), contributes to removing a conductive film andmachining the workpiece. An electric discharge pulse, the pulse width ofwhich is long and exceeds the setting value (This pulse is referred toas “a long electric discharge pulse” hereinafter.), contributes toforming a conductive film on a surface of the workpiece.

In general, it is well known that when an electric discharge pulse widthis extended in electric discharge machining in which oil is used as aworking solution, carbon produced by decomposition of the workingsolution attaches to one of the electrodes. However, in the case ofelectric discharge machining conducted on a workpiece made of insulatingmaterial or high resistance material, the above phenomenon is meaningfulfor performing the electric discharge machining. The detail of this factwill be described as follows. When a long electric discharge pulseappears, a working solution is decomposed by heat generated in theprocess of electric discharge machining. Therefore, crystalline carbon,the electric resistance of which is relatively low, is produced fromcarbon contained in the working solution, and an electric discharge faceof the workpiece is covered with this crystalline carbon. Due to theforegoing, the surface of the workpiece can be electrified, so thatelectric discharge can be continuously generated between the surface ofthe workpiece and the electrode which is a tool. The short electricdischarge pulse conducts machining on the workpiece in the same manneras that of the pulse of usual electric discharge machining, however,since electric discharge is generated with respect to a conductive filmon the surface of the workpiece produced as described above, it isestimated that the workpiece is melted or sublimed by the influence ofthe thus generated heat while the conductive film is being removed.

As described above, in the conventional electric discharge machine bywhich a workpiece made of insulating material or others is machined, along electric discharge pulse, which is meaningful for conductingelectric discharge on insulating material or others, is generated by aphenomenon which may be called a malfunction. Due to the foregoing, thewidth of a long electric discharge pulse fluctuates, and thickness of aconductive film formed on an electric discharge face of the workpiecefluctuates. As a result, electric discharge machining becomes unstableand further quality of the machined face is deteriorated.

FIG. 2 is a schematic illustration showing a composition of an electricdischarge machine of Embodiment 1 of the present invention. In thedrawing, reference numeral 1 is an electrode, reference numeral 2 a is aworkpiece made of insulating material or high resistance material, forexample, reference numeral 2 a is a workpiece made of insulatingceramics, reference numeral 14 is a DC electric power source, referencenumeral 15 is a switching element, reference numeral 16 is a resistor,reference numeral 17 is a first comparator which is a comparison memberfor comparing voltage between electrodes with predetermined referencevoltage, reference numeral 18 is a second comparator which is acomparison member for comparing voltage between electrodes withpredetermined reference voltage, and reference numeral 19 is a controlmember. The same portions as those of a conventional electric dischargemachine such as a positioning member for relatively positioning theelectrode 1 and the workpiece 2 a are omitted here. In this case, theworkpiece 2 a is coated with a conductive film in the same manner asthat described in the Japanese Unexamined Patent Publication No.Sho63-150109. Alternatively, the workpiece 2 a is contacted with andfixed to conductive material in the same manner as that described in theJapanese Unexamined Patent Publication Nos. Hei7-136849 and Hei9-253935.

FIG. 3 is a schematic illustration showing an example of a wave-form ofvoltage impressed between electrodes of an electric discharge machine ofEmbodiment 1 of the present invention. Like reference characters areused to indicate like parts in FIGS. 1 and 3. In FIG. 3, signal (a) sentfrom the control member 19 for driving the switching element 15 shown inFIG. 2, output signal (b) of the first comparator 17 and output signal(c) of the second comparator 18 are shown being synchronized with awave-form of voltage between the electrodes.

In FIG. 3, reference mark T_(P1) is a relatively short electricdischarge pulse width suitable for machining the workpiece 2 a,reference mark T_(P2) is a relatively long electric discharge pulsewidth suitable for forming a conductive film on a face of the workpiece2 a to be machined, reference mark T₀ is a predetermined period of timefrom the start of electric discharge, reference mark V₁ is referencevoltage of the first comparator 17, and reference mark V₂ is referencevoltage of the second comparator 18. Reference voltage V₁ of the firstcomparator 17 is set at a value close to electric power source voltageV₀, and reference voltage V₂ of the second comparator 18 is set at avalue lower than reference voltage V₁. Both reference voltage V₁ andreference voltage V₂ are set at values higher than arc voltage Vg. Thefirst comparator 17 compares voltage V between the electrodes withreference voltage V₁, and when reference voltage V₁ is lower thanvoltage V between the electrodes, the first comparator 17 outputs signalH, and when reference voltage V₁ is higher than voltage V between theelectrodes, the first comparator 17 outputs signal L. In the samemanner, the second comparator 18 compares voltage V between theelectrodes with reference voltage V₂, and when reference voltage V₂ islower than voltage V between the electrodes, the second comparator 18outputs signal H, and when reference voltage V₂ is higher than voltage Vbetween the electrodes, the second comparator 18 outputs signal L. Apoint of time when the output signal (b) of the first comparator 17changes from H to L is judged to be a start of electric discharge.

In the case where electric resistance of the surface of the workpiece 2a, which is made of insulating material or high resistance material, islow, like the short electric discharge pulse width T_(P1) shown in FIG.3, voltage V between the electrodes sharply drops after the start ofelectric discharge and becomes lower than reference voltage V₁ of thefirst comparator 17 and reference voltage V₂ of the second comparator18. On the other hand, in the case where electric resistance of thesurface of the workpiece 2 a is high, like the long electric dischargepulse width T_(P2) shown in FIG. 3, voltage V between the electrodesdoes not sharply drop after the start of electric discharge, but voltageV between the electrodes becomes lower than reference voltage V₁ of thefirst comparator 17, however, voltage V between the electrodes becomeshigher than reference voltage V₂ of the second comparator 18.

At a point of time when predetermined period of time T₀ has passed fromthe start of electric discharge (at the time B shown in FIG. 3), theelectric discharge pulse width is set by the control member 19 accordingto the output signals of the first comparator 17 and the secondcomparator 18. That is, in the case where electric resistance of thesurface of the workpiece 2 a is relatively low at a point of time when apredetermined period of time T₀ has passed from the start of electricdischarge, a short pulse width T_(P1) is set by the control member 19.In the case where electric resistance of the surface of the workpiece 2a is relatively high at a point of time when a predetermined period oftime T₀ has passed from the start of electric discharge, a long pulsewidth T_(P2) is set by the control member 19. The reason why the shortpulse width T_(P1) or long pulse width T_(P2) is set will be explainedbelow.

As described above, an electric discharge pulse, the pulse width ofwhich is short (The pulse width is T_(P1).), contributes to removing aconductive film and machining the workpiece 2 a. An electric dischargepulse, the pulse width of which is long (The pulse is T_(P2).),contributes to forming a conductive film on a surface of the workpiece 2a. According to the investigations made by the present inventors, thefollowing facts have been made clear. In the case where a conductivefilm is tightly formed at the electric discharge starting position ofthe workpiece 2 a, a drop in voltage is big, that is, voltage betweenthe electrodes is low, and in the case where a small quantity ofconductive film is left at the electric discharge starting position ofthe workpiece 2 a, a drop in voltage is small, that is, voltage betweenthe electrodes is high.

FIGS. 4A and 4B are schematic illustrations showing a phenomenon ofelectric discharge generated in the process of machining a workpiecemade of insulating material or high resistance material. In the drawing,reference numeral 1 is an electrode, reference numeral 2 a is aworkpiece made of insulating material or high resistance material,reference numeral 3 is an arc column, and reference numeral 20 is aconductive film. FIG. 4A is a view showing a case in which a phenomenonof electric discharge has occurred in a portion of the conductive film20 formed on the workpiece 2 a where the electric resistance is low, andFIG. 4B is a view showing a case in which a phenomenon of electricdischarge has occurred in a portion of the conductive film 20 formed onthe workpiece 2 a where the electric resistance is high. FIGS. 5A and 5Bare schematic illustrations showing a wave-form of voltage impressedbetween electrodes corresponding to FIGS. 4A and 4B. In the drawing, tis time, V is voltage impressed between the electrodes, V₀ is theelectric power source voltage, and Vg is arc voltage. FIG. 5A is a viewshowing a wave-form of voltage impressed between the electrodescorresponding to FIG. 4A, and FIG. 5B is a view showing a wave-form ofvoltage impressed between the electrodes corresponding to FIG. 4B.

In the case where voltage V between the electrodes and reference voltageare compared with each other by the comparator shown in FIG. 2, voltageof the electrode 1 and voltage on the workpiece 2 a side are comparedwith each other through the conductive film 20 as shown in FIGS. 4A and4B. In the case where the conductive film 20 is tightly formed andelectric resistance is relatively low as shown in FIG. 4A, themeasurement of voltage is substantially equivalent to a measurement inwhich an electric potential difference of the arc column 3 is directlymeasured. Therefore, voltage V between the electrode becomes equivalentto arc voltage Vg, which is about 20V to 30V as shown in FIG. 5A. On theother hand, in the case where a quantity of conductive film 20 is smalland electric resistance is high as shown in FIG. 4B, in addition to theelectric potential difference of the arc column 3, a drop in voltagecaused in a portion close to the electric discharge point, the electricresistance of which is high, is detected as voltage V between theelectrodes. Therefore, a detected value of voltage V between theelectrodes is increased as shown in FIG. 5B.

As explained above, electric resistance at the position where electricdischarge has started can be judged by the voltage immediately after thestart of electric discharge, that is, a state of the conductive film 20can be judged by the voltage immediately after the start of electricdischarge.

FIG. 6 is a schematic illustration showing an example of a change in awave-form of voltage impressed between electrodes, wherein the change inthe wave-form of voltage is caused by a state of a conductive film 20.In the drawing, t is time, V is voltage between the electrodes, and V₀is voltage of the electric power source. In FIG. 6, the following can beunderstood. According to the intensity of voltage V between theelectrodes immediately after the start of electric discharge, forexample, according to (A), (B) and (C) shown in the drawing, electricresistance of (A) at the position of start of electric discharge is thehighest, and electric resistance of (B) is next to electric resistanceof (A), and electric resistance of (C) is next to electric resistance of(B). Concerning the thickness of the conductive film 20 at the positionof start of electric discharge, (C) is the largest, and (B) is next to(A), and (C) is next to (B).

An appropriate pulse width of electric discharge, which corresponds to acase in which the thickness of the conductive film 20 is reduced asshown in FIG. 4B, may be previously found by an experiment.

For example, a state of the conductive film 20 at the point of electricdischarge is judged with the comparator shown in FIG. 2 as follows. Inthe case where the thickness of the conductive film 20 is small, thatis, in the case where voltage V between the electrodes at the point oftime when predetermined period of time T₀ has passed from the start ofelectric discharge is high, appropriate long pulse width T_(P2) ofelectric discharge is previously found by an experiment, and a state ofthe conductive film 20 at the point of electric discharge is judged fromvoltage V between the electrodes at the point of time when predeterminedperiod of time T₀ has passed from the start of electric discharge. Inthe case where voltage V between the electrodes at the point of timewhen predetermined period of time T₀ has passed from the start ofelectric discharge is low, that is, in the case where the thickness ofthe conductive film 20 is large, short pulse width T_(P1) of electricdischarge is set. In the case where voltage V between the electrodes atthe point of time when predetermined period of time T₀ has passed fromthe start of electric discharge is high that is, in the case where thethickness of the conductive film 20 is small, long pulse width T_(P2) ofelectric discharge is set. In this way, electric discharge can be madeto proceed, and an electric conductive film can be formed at a positionof electric discharge where the thickness of the conductive film isreduced, and an appropriate pulse width can be set according to thestate of the conductive film 20.

Accordingly, it is possible to solve the problems caused in theconventional electric discharge machine by which a workpiece made ofinsulating material is machined, that is, it is possible to solve theproblems that the pulse width of a long electric discharge pulse is notstabilized and the thickness of a conductive film formed on the electricdischarge face of the workpiece fluctuates. As a result, the workpiecemade of insulating material or high resistance material can be stablymachined, and quality of the machined face can be enhanced.

For example, in the case of electric discharge conducted by a diesinkingelectric discharge machine of the prior art, surface roughness on amachined face of a workpiece is approximately 10 μm, however, accordingto electric discharge machining conducted by a diesinking electricdischarge machine of the present invention, surface roughness on amachined face of a workpiece can be improved to about 3 μm.

In the above explanation, for example, as shown in FIG. 2, the twocomparators are provided, the start of electric discharge is judged byan output signal of the first comparator 17, electric resistance on thesurface of the workpiece is judged by an output signal of the secondcomparator 18, and two types of the electric discharge pulse width areset according to the electric resistance. However, the number of thecomparators may be increased, that is, three or more types of theelectric discharge pulse width may be set. In the case where three ormore types of electric discharge pulse width are set according toelectric resistance between the electrodes, it becomes possible to moreprecisely set the electric discharge pulse width according to a state ofthe conductive film attached to the workpiece. Therefore, the workpiecemade of insulating material or high resistance material can be morestably conducted, and quality of the machined face can be enhanced.

In this connection, the electric discharge machining method and electricdischarge machine of the present invention can be applied to wireelectric discharge machining, diesinking electric discharge machiningand small hole machining.

As explained above, according to the electric discharge machining methodand electric discharge machine of the present invention, in the electricdischarge machining of a workpiece made of insulating material or highresistance material, a state of the surface of the workpiece is judgedfrom voltage between the electrodes which changes by electric resistanceon the surface of the workpiece, and the width of a long electricdischarge pulse for forming a thick conductive film out of a thinconductive film on the workpiece is appropriately set according to theresult of the judgment. Therefore, the present invention is differentfrom the invention disclosed in the Japanese Unexamined PatentPublication No. Hei3-3722 in which the pulse width of electric dischargeis adjusted in order to conduct machining while electric dischargeenergy is being kept constant according to a detected value of voltageimpressed between the electrodes.

Embodiment 2

FIG. 7 is a schematic illustration showing a composition of an electricdischarge machine of Embodiment 2 of the present invention. Likereference characters are used to indicate like parts in FIG. 2 showingEmbodiment 1 and FIG. 7 showing Embodiment 2. In FIG. 7, referencenumeral 21 is a voltmeter which is a measurement member for measuringvoltage impressed between the electrodes, and the thus measured value issent to the control member 19. In the case where voltage between theelectrodes measured by the voltmeter 21 becomes a value not higher thanthe first reference voltage, which is voltage that has been set at avalue close to the electric power source voltage not higher than theelectric power source voltage, it is judged that electric discharge hasstarted.

At a point of time when a predetermined period of time (for example, aperiod of time corresponding to T₀ shown in FIG. 3) has passed, a signalis sent from the control member 19 to the voltmeter 21. At this time,the voltmeter 21 measures volt impressed between the electrodes. In thiscase, as shown Embodiment 1, an attaching state of the conductive filmcan be judged by the electric resistance between the electrodes.Therefore, according to the value of voltage between the electrodesmeasured by the voltmeter 21, this measured value and the secondreference voltage, which is voltage to be set at a value not higher thanthe first reference voltage and higher than the arc voltage, arecompared with each other. In the case where the measured value is lowerthan the second reference voltage, a relatively short electric dischargepulse width (for example, T_(P1) shown in FIG. 3), which is appropriatefor machining the workpiece 2 a, is set by the control member 19. In thecase where the measured value is higher than the second referencevoltage, a relatively long electric discharge pulse width (for example,T_(P2) shown in FIG. 3), which is appropriate for forming a conductivefilm on the machined face of the workpiece 2 a, is set by the controlmember 19.

When the above composition is adopted, the same effect as that ofEmbodiment 1 can be provided.

INDUSTRIAL APPLICABILITY

As described above, the electric discharge machining method and electricdischarge machine of the present invention is appropriately used forelectric discharge machining of a workpiece made of insulating materialor high resistance material.

1. In an electric discharge machining method for machining a workpiecewhen machining electric power is supplied between said workpiece, whichis made of insulating material or high resistance material coated with aconductive film, and an electrode, or machining electric power issupplied between said workpiece, which is made of insulating material orhigh resistance material coming into contact with conductive material,and an electrode so that pulse-like electric discharge is generatedbetween said electrodes in a working solution containing carbon, saidelectric discharge machining method comprising the steps of: a voltagebetween said electrodes at a point of time when a predetermined periodof time has passed from the start of electric discharge is measured; anda relatively short electric discharge pulse width suitable for machiningsaid workpiece according to the measured value is set and a relativelylong electric discharge pulse width suitable for forming a conductivefilm on a surface of said workpiece to be machined is set.
 2. In anelectric discharge machining method for machining a workpiece whenmachining electric power is supplied between said workpiece, which ismade of insulating material or high resistance material coated with aconductive film, and an electrode, or machining electric power issupplied between said workpiece, which is made of insulating material orhigh resistance material coming into contact with conductive material,and electrode so that pulse-like electric discharge is generated betweensaid electrodes in a working solution containing carbon, said electricdischarge machining method comprising the steps of: a voltage betweensaid electrodes at a point of time when a predetermined period of timehas passed from the start of electric discharge is measured; and arelatively short electric discharge pulse width suitable for machiningsaid workpiece in the case where the measured value is lower than apredetermined reference voltage is set, and a relatively long electricdischarge pulse width suitable for forming a conductive film on a faceof said workpiece to be machined in the case where the measured value ishigher than a predetermined reference voltage is set.
 3. In an electricdischarge machining method for machining a workpiece when machiningelectric power is supplied between said workpiece, which is made ofinsulating material or high resistance material coated with a conductivefilm, and an electrode, or machining electric power is supplied betweensaid workpiece, which is made of insulating material or high resistancematerial coming into contact with conductive material, and electrode sothat pulse-like electric discharge is generated between said electrodesin a working solution containing carbon, said electric dischargemachining method comprising the steps of: at least one referencevoltage, which is lower than an electric power source voltage, with avoltage between said electrodes is compared; and a relatively shortelectric discharge pulse width suitable for machining said workpieceaccording to the result of the comparison is set, and a relatively longelectric discharge pulse width suitable for forming a conductive film ona surface of said workpiece to be machined according to the result ofthe comparison is set.
 4. In an electric discharge machining method formachining a workpiece when machining electric power is supplied betweensaid workpiece, which is made of insulating material or high resistancematerial coated with a conductive film, and an electrode, or machiningelectric power is supplied between said workpiece, which is made ofinsulating material or high resistance material coming into contact withconductive material, and electrode so that pulse-like electric dischargeis generated between said electrodes in a working solution containingcarbon, said electric discharge machining method comprising the stepsof: a voltage between said electrodes at a point of time when apredetermined period of time has passed from the time when the voltagebetween said electrodes becomes lower than a first reference voltagewhich is set at a value close to the electric power source voltage lowerthan the electric power source voltage is measured; and a relativelyshort electric discharge pulse width suitable for machining saidworkpiece in the case where the measured value is lower than apredetermined second reference voltage is set, and a relatively longelectric discharge pulse width suitable for forming a conductive film ona face of said workpiece to be machined in the case where the measuredvalue is higher than a predetermined second reference voltage is set. 5.An electric discharge machine for machining a workpiece by generatingpulse-like electric discharge between an electrodes comprising: amachining electric power supply member for supplying machining electricpower between a workpiece made of insulating material or high resistancematerial which are coated with a conductive film, and an electrode, or amachining electric power supply member for supplying machining electricpower between said workpiece made of insulating material or highresistance material which are coming into contact with conductivematerial, and an electrode; a machining liquid supply member forsupplying a machining liquid containing carbon between said electrodes;a positioning member for relatively positioning said workpiece and saidelectrode; a voltage measuring member for measuring voltage between saidelectrodes at a point of time when a predetermined period of time haspassed from the start of electric discharge; and a control member forsetting a relatively short electric discharge pulse width suitable formachining said workpiece according to the measured value of voltagebetween said electrodes measured by the voltage measuring member formeasuring voltage between said electrodes and for setting a relativelylong electric discharge pulse width suitable for forming a conductivefilm on a surface of said workpiece to be machined.
 6. An electricdischarge machine for machining a workpiece by generating pulse-likeelectric discharge between an electrodes comprising: a machiningelectric power supply member for supplying machining electric powerbetween a workpiece made of insulating material or high resistancematerial which are coated with a conductive film, and an electrode, or amachining electric power supply member for supplying machining electricpower between said workpiece made of insulating material or highresistance material which are coming into contact with conductivematerial, and an electrode; a machining liquid supply member forsupplying a machining liquid containing carbon between said electrodes;a positioning member for relatively positioning said workpiece and saidelectrode; a voltage measuring member for measuring voltage between saidelectrodes at a point of time when a predetermined period of time haspassed from the start of electric discharge; and a control member forsetting a relatively short electric discharge pulse width suitable formachining said workpiece in the case where the measured value is lowerthan a predetermined reference voltage according to the measured valueof voltage between said electrodes measured by the voltage measuringmember for measuring voltage between said electrodes, and for setting arelatively long electric discharge pulse width suitable for forming aconductive film on a face of said workpiece to be machined in the casewhere the measured value is higher than a predetermined referencevoltage according to the measured value of voltage between saidelectrodes measured by the voltage measuring member for measuringvoltage between said electrodes.
 7. An electric discharge machine formachining a workpiece by generating pulse-like electric dischargebetween an electrodes comprising: a machining electric power supplymember for supplying machining electric power between a workpiece madeof insulating material or high resistance material which are coated witha conductive film, and an electrode, or a machining electric powersupply member for supplying machining electric power between saidworkpiece made of insulating material or high resistance material whichare coming into contact with conductive material, and an electrode; amachining liquid supply member for supplying a machining liquidcontaining carbon between said electrodes; a positioning member forrelatively positioning said workpiece and said electrode; a voltagemeasuring member for measuring voltage between said electrodes; and acontrol member for setting a relatively short electric discharge pulsewidth suitable for machining said workpiece in the case where a measuredvalue (second measured value), which is measured by the voltagemeasuring member for measuring voltage between said electrodes at apoint of time when a predetermined period of time has passed from thetime when a measured value (first measured value) measured by thevoltage measuring member for measuring voltage between said electrodesbecomes a value lower than a first reference voltage which is set closeto electric power source voltage not higher than the electric powersource voltage, is lower than a predetermined second reference voltage,and for setting a relatively long electric discharge pulse widthsuitable for forming a conductive film on a face of said workpiece inthe case where the second measured value is higher than thepredetermined reference voltage.
 8. An electric discharge machine formachining a workpiece by generating pulse-like electric dischargebetween an electrodes comprising: a machining electric power supplymember for supplying machining electric power between a workpiece madeof insulating material or high resistance material which are coated witha conductive film, and an electrode, or a machining electric powersupply member for supplying machining electric power between saidworkpiece made of insulating material or high resistance material whichare coming into contact with conductive material, and an electrode; amachining liquid supply member for supplying a machining liquidcontaining carbon between said electrodes; a positioning member forrelatively positioning said workpiece and said electrode; at least onecomparison member for comparing a reference voltage, which is lower thanthe electric power source voltage, with the voltage between saidelectrodes; and a control member for setting a relatively short electricdischarge pulse width suitable for machining said workpiece according tothe result of the comparison, and for setting a relatively long electricdischarge pulse width suitable for forming a conductive film on a faceof said workpiece to be machined.
 9. An electric discharge machine formachining a workpiece by generating pulse-like electric dischargebetween an electrodes comprising: a machining electric power supplymember for supplying machining electric power between a workpiece madeof insulating material or high resistance material which are coated witha conductive film, and an electrode, or a machining electric powersupply member for supplying machining electric power between saidworkpiece made of insulating material or high resistance material whichare coming into contact with conductive material, and an electrode; amachining liquid supply member for supplying a machining liquidcontaining carbon between said electrodes; a positioning member forrelatively positioning said workpiece and said electrode; a comparisonmember for comparing voltage between said electrodes at a point of timewhen a predetermined period of time has passed from the start ofelectric discharge with predetermined reference voltage; and a controlmember for setting a relatively short electric discharge pulse widthsuitable for machining said workpiece in the case where the voltagebetween said electrodes is lower than the reference voltage according tothe result of the comparison conducted by the comparison member and acontrol member for setting a relatively long electric discharge pulsewidth suitable for forming a conductive film on a face of said workpiecein the case where the voltage between said electrodes is higher than thereference voltage.
 10. An electric discharge machine for machining aworkpiece by generating pulse-like electric discharge between anelectrodes comprising: a machining electric power supply member forsupplying machining electric power between a workpiece made ofinsulating material or high resistance material which are coated with aconductive film, and an electrode, or a machining electric power supplymember for supplying machining electric power between said workpiecemade of insulating material or high resistance material which are cominginto contact with conductive material, and an electrode; a machiningliquid supply member for supplying a machining liquid containing carbonbetween said electrodes; a positioning member for relatively positioningsaid workpiece and said electrode; a first comparison member forcomparing voltage between said electrodes with a first reference voltagewhich is set at a value close to the electric power source voltage nothigher than the electric power source voltage; a second comparisonmember for comparing voltage between said electrodes with a secondreference voltage which is set at a value lower than the first referencevoltage; and a control member for setting a relatively short electricdischarge pulse width suitable for machining said workpiece in the casewhere the voltage between said electrodes is lower than the secondreference voltage according to the result of the comparison conducted bythe second comparison member at a point of time when a predeterminedperiod of time has passed from the time at which the voltage betweensaid electrodes becomes lower than the first reference voltage accordingto the result of the comparison conducted by the first comparisonmember, and for setting a relatively long electric discharge pulse widthsuitable for forming a conductive film on a machined face of saidworkpiece in the case where the voltage between said electrodes ishigher than the second reference voltage.